Implantable medical devices are being increasingly used to treat medical conditions. These types of devices include pacemakers, implantable cardioverter defibrillators, insulin pumps and the like. Being implanted, these devices are capable of providing therapy directly to the patient on a periodic or continuous basis. These devices are typically battery powered and are susceptible to battery failure over time. One class of commonly used implantable medical devices is implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter defibrillators.
Implantable cardiac stimulation devices are typically implanted within the body cavity of a patient to monitor heart activity and provide therapeutic stimuli to treat a variety of medical conditions. Typically, cardiac stimulation devices monitor the beating of the heart and provide artificial stimulation to the heart to override brady/tachycardia and other arrhythmias as well as to interrupt fibrillation. These devices generally include circuitry for sensing heart function as well as circuitry and sensors that detect physiologic conditions of the patient. Conventionally, implantable cardiac stimulation devices comprise a variety of electronic components such as a limited power source, e.g. a battery, sensing circuits, stimulation circuits, a microcontroller, resistors, and capacitors, which are encapsulated within a small, biocompatible, hermetically sealed enclosure which protects the internal circuitry of the implantable cardiac stimulation device.
Once the implantable cardiac stimulation device has been assembled and is ready for patient implantation, the device is powered-up, and the individual circuit components of the device begin consuming battery power. This consumption of battery power often begins prior to actual implantation as the device is often enabled during final assembly. This power up of the device prior to implantation can significantly reduce the overall effective life of the device. Ultimately, the overall useful life of the cardiac stimulation device for the end user is minimized due to a reduced battery life and an inefficient technique of power consumption by the device. As a result, the implantable cardiac stimulation device has a limited shelf life prior to patient implantation.
Implantation of the cardiac stimulation device is an invasive procedure that often involves surgical entry into the body cavity of the patient. Such invasive surgical implantation procedures are physically demanding for the patient. Consequently, replacement of the implanted device due to impending battery failure results in the risk and discomfort to the patient. Therefore, extending the useful life of the implantable cardiac stimulation device would significantly reduce the amount of surgeries that a patient would undergo just to replace exhausted batteries.
Another aspect of implantable cardiac stimulation devices is that they may be designed and equipped with the ability to selectively activate a particular feature, function, or mode of operation with the use of a magnetic field sensor switch. Traditionally, these magnetic sensor devices employ a known reed switch that can be mechanically closed with the exposure of the reed switch to a magnetic field of a given threshold value. For example, one method of retrieving information recorded by the cardiac stimulation device is by a known wireless communication link, such as a telemetry circuit, from the implanted device to an external receiver, wherein an integrated telemetry circuit is utilized to transmit recorded monitoring data through body tissue to an external receiver via radio frequency transmission. Typically, the telemetry circuit is enabled through the use of a reed switch. This allows a medical professional to activate the downloading of the data stored in the device by positioning a magnet outside the patient's body proximate the implanted device. The reed switch is then triggered which results in a signal being sent to the implanted device enabling the telemetry circuitry.
Unfortunately, reed switches can be unreliable in operation. Typically, reed switches employ mechanical contacts, wherein mechanical failure is induced by a contact bounce, which may result in false contact readings. In addition, due to the function of the switch relying upon a mechanical occurrence, i.e., the mechanical movement of the reeds in response to the applied external magnetic field, variations in the material comprising the reeds may result in variations of performance of the reed switch. Hence, in some circumstances, the application of an external magnetic field may not result in the actuation of the reed switch in the desired fashion. Moreover, the repeated actuation of the reed switch may result in mechanical fatigue in at least one of the reeds to the point where the reed may become unreliable and inaccurate. Additionally, the reeds may occasionally remain stuck together after the application of the external magnetic field. In this circumstance, the reed switch may be continuously inducing the microcontroller to perform a specific function that is not needed thus inadvertently consuming power. This can also result in the reed switch no longer being capable of sensing future applications of the applied magnetic field. Furthermore, the mechanical movement required to provide a connection limits the overall switching effectiveness, response time, and reaction speed.
Moreover, reed switches are inherently designed to sense magnetic fields in any direction or magnitude thereof. Consequently, the reed switch may be accidentally triggered. This is the result of the inability of the reed switch to distinguish between magnetic fields of different magnitudes, when the magnitude is greater than an initial sensitivity threshold of the reed switch. Specifically, the patient may inadvertently be in a strong magnetic field, which could trigger the switch. For example, the strong magnetic field may be the result of the patient being around industrial equipment or undergoing a medical procedure such as magnetic resonance imaging (MRI). The inadvertent closing of the reed switch, in such a circumstance, may result in the implantable cardiac stimulation device switching to an undesired mode of operation or initiating an undesired function, which may also inadvertently consume power.
From the foregoing, there is a need for a device that permits activation of implantable medical devices, or activation of functions performed by implantable medical devices in a manner that is more reliable. To this end, there is a need for a device that allows for activation only when a specific magnetic field is detected and that is less likely to be inadvertently activated.